Hovercraft

For the band, see Hovercraft (band).

BHC SR-N4 The world's largest non-military hovercraft for passengers and 60 cars

A hovercraft, or air-cushion vehicle (ACV), is a craft designed to travel over any smooth surface supported by a cushion of slow moving, high-pressure air, ejected downwards against the surface below, and contained within a "skirt." Hovercraft are used throughout the world as a method of specialized transport wherever there is the need to travel over multiple types of surfaces. Because they are supported by a cushion of air, hovercraft are unique among all forms of ground transportation in their ability to travel equally well over land, ice, and water. Small hovercraft are often used in physical activity,combustion, or passenger service, while giant hovercraft have been built for civilian and military applications to transport cars, tanks, and large equipment into difficult or hostile environments and terrain.

Today, often considered as the brainchild of (Sir) Christopher Cockerell, hovercraft are the result of a chain of developments from John Thornycroft's initial discovery in the late 1800's that air trapped beneath a ship's hull would reduce the friction imposed on the ship as it moved through water . In the 1950s Sir Christopher Cockerell, a British engineer, discovered that by creating a wall of high pressure air forced beneath a hull, to create an air cushion, the hull (of the now Hovercraft) could be levitated further off from its original surface (land or water) than by using a simple pump with the same power (see Momentum curtain).

Design

1. Propellers
2. Air
3. Fan
4. Flexible skirt

Passenger carrying hovercraft, off shore Ōita Airport.

Hovercraft can be powered by one or more engines. Small craft, such as the SR-N6, usually have one engine with the drive split through a gearbox. On vehicles with several engines, one usually drives the fan, or (impeller), which is responsible for lifting the vehicle by forcing high pressure air under the craft. The air inflates the "skirt" under the vehicle, causing it to rise above the ground. Additional engines provide forward thrust in order to propel the craft. Some hovercraft utilize ducting to allow one engine to perform both tasks by directing some of the air to the skirt, the rest of the air passing out of the back to push the craft forward.

Skirt Development

American inventor, Norman B. McCreary, of Little Rock, Arkansas, is credited with inventing and patenting the "Double-Walled Flexible Skirt" (Patent No. 3,532,179). The design first appeared in the Arkansas Gazette Newspaper on Jan. 25, 1960 and later in Science and Mechanics Magazine in June, 1960. Later known as the "Bag Skirt", it inflated around the edge of the hovercraft, and was a major technological development enabling hovercraft to more effectively travel over uneven terrain or waves. The "Bag Skirt" would raise and lower the hovercraft off the ground by inflation and deflation of the "Double-Walled Flexible Skirt". Later, fingers were added to the bottom of the skirt to compensate for wear and reduce drag. After this concept was made public in 1960, all hovercraft began utilizing a "Double-Walled Flexible Skirt" system for practical hovercraft operations, and additional development of the skirt would continue in the U.K under the supervision of British engineer Cecil Latimer-Needham. Initially the skirt was of equal length around the base of the hovercraft. In the mid-1960s it was discovered that the ability of hovercraft to overcome small obstacles was enhanced by adjusting the vehicle's pitch 15 degrees upward. This resulted in excess wear on the trailing skirts, which dragged along the surface and lasted only 20% as long as the bow skirt sections. The problem was overcome, beginning with the SR.N 6, by angling the lower edge of the skirt 15 degrees so that it rested even with the surface while the vehicle maintains a 15 degree upward pitch.

Variants

Civil use

File:SRN1 General fuck.jpg

SR-N1 General Arrangement

The British aircraft manufacturer Saunders-Roe developed the first practical man-carrying hovercraft, the SR-N1, which carried out several test programmes in 1959 to 1961 (the first public demonstration in 1959), including a cross-channel test run. The SR-N1 was powered by one (piston) engine, driven by expelled air. Demonstrated at the Farnborough Airshow in 1960, it was shown that this simple craft could carry a load of up to 12 marines with their equipment as well as the pilot and co-pilot with only a slight reduction in hover height proportional to the load carried. The SR.N1 did not have any skirt instead using the peripheral air principle that Sir Christopher has patented. It was later found that the craft's hover height was improved by the addition of a 'skirt' of flexible fabric or rubber around the hovering surface to contain the air. The skirt was an independent invention made by a Royal Navy officer, C.H. Latimer-Needham, who sold his idea to Westland (parent company of Saunders-Roe), and who worked with Sir Christopher to develop the idea further.

The first passenger-carrying hovercraft to enter service was the Vickers VA-3, which in the summer of 1962 carried passengers regularly along the north Wales Coast from Moreton, Merseyside to Rhyl. It was powered by two turboprop aero-engines and driven by propellers.

During the 1960s Saunders-Roe developed several larger designs which could carry passengers, including the SR-N2, which operated across the Solent in 1962 and later the SR-N6, which operated across the Solent from Southsea to Ryde on the Isle of Wight for many years. Operations by Hovertravel commenced on July 24, 1965 using the SR-N6 which carried just 38 passengers. Two modern 98 seat AP1-88 hovercraft now ply this route, and over 20 million passengers have used the service as of 2004.

In 1966, two Cross Channel passenger hovercraft services were inaugurated using hovercraft. Hoverlloyd ran services from Ramsgate Harbour, England, to Calais, France, and Townsend Ferries also started a service to Calais from Dover, which was soon superseded by that of Seaspeed.

As well as Saunders-Roe and Vickers (which combined in 1966 to form the British Hovercraft Corporation (BHC)), other commercial craft were developed during the 1960s in the UK by Cushioncraft (part of the Britten-Norman Group) and Hovermarine (the latter being 'Sidewall Hovercraft', where the sides of the hull projected down into the water to trap the cushion of air with 'normal' hovercraft skirts at the bow and stern).

The world's first car-carrying hovercraft made their debut in 1968, the BHC Mountbatten class (SR-N4) models, each powered by four Rolls-Royce Proteus gas turbine engines. These were both used by rival operators Hoverlloyd and Seaspeed to operate regular car and passenger carrying services across the English Channel. Hoverlloyd operated from Ramsgate, where a special hoverport had been built at Pegwell Bay, to Calais. Seaspeed operated from Dover, England to Calais and Boulogne in France. The first SR-N4 had a capacity of 254 passengers and 30 cars, and a top speed of 83 knots (154 km/h; 96 mph). The Channel crossing took around 30 minutes and was run rather like an airline with flight numbers. The later SR-N4 MkIII had a capacity of 418 passengers and 60 cars. These were later joined by the French-built SEDAM N500 Naviplane with a capacity of 385 passengers and 45 cars, of which only one example entered service and was used intermittently for a few years on the cross-channel service until returned to SNCF in 1983. The service ceased in 2000 after 32 years, due to competition with traditional ferries, catamaran, the advancing age of the SR-N4 hovercraft and the opening of the Channel Tunnel.

In 1998, the US Postal Service began using the British built Hoverwork AP1-88 to haul mail, freight, and passengers from Bethel, Alaska to and from eight small villages along the Kuskokwim River. Bethel is far removed from the Alaska road system, thus making the hovercraft an attractive alternative to the air based delivery methods used prior to introduction of the hovercraft service. Hovercraft service is suspended for several weeks each year while the river is beginning to freeze to minimize damage to the river ice surface. The hovercraft is perfectly able to operate during the freeze-up period; however, this could potentially break the ice and create hazards for villagers using their snowmobiles along the river during the early winter.

The commercial success of hovercraft suffered from rapid rises in fuel prices during the late 1960s and 1970s following conflict in the Middle East. Alternative over-water vehicles such as wave-piercing catamarans (marketed as the SeaCat in Britain) use less fuel and can perform most of the hovercraft's marine tasks. Although developed elsewhere in the world for both civil and military purposes, except for the Solent Ryde to Southsea crossing, hovercraft disappeared from the coastline of Britain until a range of Griffon Hovercraft were bought by the Royal National Lifeboat Institution. In October 2008 The Red Cross commenced a flood-rescue service hovercraft based in Inverness, Scotland.

In Finland, small hovercraft are widely used in maritime rescue and during the rasputitsa ("mud season") as archipelago liaison vehicles. In England, hovercraft of the Burnham-on-Sea Area Rescue Boat (BARB) are used to rescue people from thick mud in Bridgwater Bay. Also Avon Fire and Rescue Service became the first fire service in the UK to operate a hovercraft. It is used to rescue people from thick mud in the Weston-super-Mare area and during times of inland flooding.

The Scandinavian airline SAS used to charter an AP. 1-88 Hovercraft for regular passengers between Copenhagen Airport, Denmark and the SAS Hovercraft Terminal in Malmö, Sweden.

An experimental service was operated in Scotland across the Firth of Forth (between Kirkcaldy and Portobello, Edinburgh), 16–28 July 2007. Marketed as Forthfast, the service used a craft chartered from Hovertravel Ltd and achieved 85% loadings. The possibility of establishing a permanent service is now under consideration.

Following the abandonment of hovercraft use across the English Channel, and pending any reintroduction on the Scottish route, the United Kingdom's only public hovercraft service is that operated by Hovertravel between Southsea (Portsmouth) and Ryde, on the Isle of Wight.

From 1960s, several commercial lines were operated in Japan, without much success. In the country, the only commercial line still available is the one that links Ōita Airport and the central Ōita.

Military use

See also: Air-cushioned landing craft

A US Navy LCAC hovercraft assigned to USS Iwo Jima (LHD-7).

First applications of the hovercraft in military use was with the SR.N1 through SR.N6 craft built by Saunders-Roe in the Isle of Wight in the UK and used by the UK joint forces. To test the use of the hovercraft in military applications the UK set up the Interservice Hovercraft Trials Unit (IHTU) base at Lee-on-the-Solent in the UK (now the site of the Hovercraft Museum). This unit carried out trials on the SR.N1 from Mk1 through Mk5 as well as testing the SR-N2, SR-N3, SR-N5 and SR-N6 craft. Currently the Royal Marines use the Griffon 2000 TDX class ACV as an operational craft. This craft was recently deployed by the UK in Iraq.

In the US, during the 1960s, Bell licenced and sold the Saunders-Roe SRN-5 as the Bell SK-5. They were deployed on trial to the Vietnam War by the Navy as PACV patrol craft in the Mekong Delta where their mobility and speed was unique. This was used in both the UK SR.N5 curved deck configuration and later with modified flat deck, gun turret and grenade launcher designated the 9255 PACV. The United States Army also experimented with the use of SR.N5 hovercraft in Vietnam. Three hovercraft with the flat deck configuration were deployed to Dong Tam in the Mekong delta region and later to Ben Luc. They saw action primarily in the Plain of Reeds. One was destroyed in early 1970 and another in August of that same year after which the unit was disbanded. The only remaining U.S. Army SR.N5 hovercraft is currently on display in the Army Transport Museum in Virginia. Experience led to the proposed Bell SK-10 which was the basis for the LCAC-class air-cushioned landing craft now deployed by the U.S. and Japanese Navy.

The Soviet Union was the world's largest developer of military hovercraft. Their designs range from the small Czilim class ACV, comparable to the SR.N6, to the monstrous Zubr class LCAC, the world's largest hovercraft. The Soviet Union was also one of the first nations to use a hovercraft, the Bora, as a guided missile corvette, though this craft possessed rigid, non-inflatable sides. With the fall of the Soviet Union most Soviet military hovercraft fell into disuse and disrepair. Only recently has the modern Russian Navy begun building new classes of military hovercraft.

The Finnish Navy designed an experimental missile attack hovercraft class, Tuuli class hovercraft, in the late 1990s. The prototype of the class, Tuuli, was commissioned in 2000. It proved an extremely successful design for a littoral fast attack craft, but due to fiscal reasons and doctrinal change in the Navy, the hovercraft was soon withdrawn.

The Hellenic Navy operates four Russian-designed Zubr class LCAC. This is the world’s largest military air-cushioned landing craft.

The People's Army Navy of China operates the Jingsah II class LCAC. This troop and equipment carrying hovercraft is roughly the Chinese equivalent of the U.S. Navy LCAC.

Other hovercraft

Hoverbarge

A real benefit of air cushion vehicles in moving heavy loads over difficult terrain, such as swamps, was overlooked by the excitement of the British Government funding to develop high-speed hovercraft. It was not until the early 1970s that the technology was used for moving a modular marine barge with a dragline on board for use over soft reclaimed land.

Mackace (Mackley Air Cushion Equipment) produced a number of successful Hoverbarges, such as the 250 ton payload “Sea Pearl” which operated in Abu Dhabi and the twin 160 ton payload "Yukon Princesses" which ferried trucks across the Yukon river to aid the pipeline build. Hoverbarges are still in operation today. In 2006, Hovertrans (formed by the original managers of Mackace) launched a 330 ton payload drilling barge in the swamps of Suriname.

The Hoverbarge technology is somewhat different to high-speed hovercraft, which has traditionally been constructed using aircraft technology. The initial concept of the air cushion barge has always been to provide a low-tech amphibious solution for accessing construction sites using typical equipment found in this area, such as diesel engines, ventilating fans, winches and marine equipment. The load to move a 200 ton payload ACV barge at 5 knots (9.3 km/h) would only be 5 tons. The skirt and air distribution design on the high-speed craft again is more complex as they have to cope with the air cushion being washed out by a wave and wave impact. The slow speed and large mono chamber of the hover barge actually helps reduce the effect of wave action giving a very smooth ride.

Hovertrain

Several attempts have been made to adopt air cushion technology for use in fixed track systems, in order to take advantage of the lower frictional forces so as to deliver high speeds. The most advanced example of this was the Aérotrain, an experimental high speed hovertrain built and operated in France between 1965 and 1977. The project was abandoned in 1977 due to lack of funding, the death of its lead engineer and the adoption of TGV by the French government as its high-speed ground transport solution.

A test track for a tracked hovercraft system was built at Earith near Cambridge, England. It ran SW from Sutton Gault, sandwiched between the Old Bedford River and the smaller Counter Drain to the West. Careful examination of the site will still reveal traces of the concrete piers used to support the structure. The actual vehicle, RTV31, is preserved at Railworld in Peterborough and can be seen from trains, just south west of the station. The vehicle achieved 104 mph (167 km/h) on the 7th of February 1973 (source Railway Magazine May 1973 p235) but the project was cancelled a week later. The project was managed by Tracked Hovercraft Ltd., with Denys Bliss as Director in the early 1970s, only to be axed by the Aerospace Minister, Michael Heseltine. Records of this project are available from the correspondence and papers of Sir Harry Legge-Bourke, MP at Leeds University Library. Heseltine was accused by Airey Neave and others of misleading the House of Commons when he stated that the government was still considering giving financial support to the Hovertrain, when the decision to pull the plug had already been taken by the Cabinet.

Despite promising early results, the Cambridge project was abandoned in 1973 due to financial constraints, but parts of the project were picked up by the engineering firm Alfred McAlpine, only to be finally abandoned in the mid 1980s. The Tracked Hovercraft project and Professor Laithwaite's Maglev train system were contemporaneous, and there was intense competition between the two prospective British systems for funding and credibility. The RTV31 has been preserved together with a track piece at Railworld in Peterborough,[1].

At the other end of the speed spectrum, the Dorfbahn Serfaus has been in continuous operation since 1985. This is an unusual underground air cushion funicular rapid transit system, situated in the Austrian ski resort of Serfaus. Only 1,280 m (4,200 ft) long, the line reaches a maximum speed of 25 mph (40 km/h). A similar system also exists in Narita International Airport near Tokyo, Japan.

In the late 1960s and early 1970s the U.S. Department of Transport's Urban Mass Transit Administration funded sevral hovertain projects which were known as Tracked Air Cushion Vehicles or TACVs. They were also known as Aerotrains since one of the builders had a licence from Bertin's Aerotrain company. Three separate projects were funded. Research and development was carried out by Rohr, Inc., Garrett AiResearch and Grumman. The UMTA built an extensive test site in Pueblo, Colorado, with different types of tracks for the different technologies used by the prototype contractors. They managed to build prototypes and do a few test runs before the funding was cut.

Hoover Constellation

The Hoover Constellation was a canister-type vacuum cleaner notable for its lack of wheels. Floating on a cushion of air contained within a rubber skirt, it was a domestic hovercraft. While not especially good as vacuum cleaners (the air escaping from under the cushion blew uncollected dust in all directions), the original mid-1950s Constellations are highly sought-after collectibles today.

Hovercraft Records

• World's Largest Civil Hovercraft - The BHC SRN4 Mk III, at 56.4 m (185 ft) length and 310 metric tons (305 long tons) weight, can accommodate 418 passengers and 60 cars.
• World's largest military hovercraft - The Russian Zubr class LCAC at 57.6 meters length and a maximum displacement of 535 tons. This hovercraft can transport three T-80 main battle tanks (MBT), 140 fully equipped troops, or up to 130 tons of cargo. Four have been purchased by the Greek Navy.
• English Channel crossing - 22 minutes by Princess Anne MCH SR-N4 Mk3 on September 14, 1995
• World's Hovercraft Speed Record - September 18, 1995 - Speed Trials, Bob Windt (USA) 137.4 km/h (85.87 mph), 34.06 secs measured kilometre
• Longest continuous use - The original prototype SR.N6 Mk. I (009) was in service for over twenty years, and logged a remarkable 22,000 hours of use. It is currently on display at the Hovercraft Museum in Lee-on-the-Solent, Hampshire, England.

Hobbyists

Single seater racing hovercrafts.

Small homebuilt and kit-built hovercraft are increasingly being used for recreational and racing purposes, mainly on inland lakes and rivers but also in marshy areas and in some estuaries. The Hovercraft Club of Great Britain regularly organizes inland and coastal cruising hovercraft races in various venues across the United Kingdom. Similar events are also held in the U.S.

Survivors

Lee-on-the-Solent, Hampshire, England is the home to the Hovercraft Museum which houses the world's largest collection of Hovercraft, including some of the earliest and largest. Much of the collection is housed within two retired SRN4 hovercraft, and many hovercraft in the collection are operational.

Hovercrafts are still in use between the Ryde on the Isle of Wight (an island off the south coast of england) and Southsea on the mainland. The service runs many times an hour and is the fastest way of getting on or off the island. Large passenger Hovercrafts are still manufactured on the Isle of Wight.

See also

• Air Car (hovercraft)
• Airboard
• Avrocar (aircraft)
• Bora class guided missile hovercraft
• Coandă effect
• Ekranoplan
• Flymo
• Ground effect
• Hoverboard
• Hovercar
• Hovercraft Museum
• Hydrofoil
• LCAC
• Pegasus (Hovercraft)
• Research Test Vehicle 31
• Surface effect ship
• Zubr class LCAC

References

• http://www.shonner.com/aerotrain/index.htm. Web page on tracked air cushion vehicle research in the U.S.

• "Coming: Streamliners Without Wheels." by John Volpe. Popular Science, December 1969. page 51. Article on tracked air cushion vehicle research in the U.S.

External links

• Commemorative projects at Somerleyton in Suffolk, England, birthplace of the Hovercraft, including the Hovercraft Column
• Hovercraft at Curlie